I've got mixed feelings with the teen idol stuff. I've seen some Japanese parents push things too far with their kids into the sexual zone. On the other hand you are correct with things like Babymetal. Doing the teen idol thing pretty much is the starting steps to becoming an idol, actor, voice actor, singer, or a few other jobs within the entertainment sector of Japan. It's very competitive when it comes to working the entertainment industry in Japan so starting young does give you a leg up in all of that.